The present invention relates to a rotary anode type X-ray tube and an x-ray tube apparatus provided with the same, particularly, to a rotary anode type X-ray tube equipped with a hydrodynamic type slide bearing having a spiral groove and an X-ray tube apparatus having the rotary anode type X-ray tube incorporated therein.
A rotary anode type X-ray tube comprises a rotary anode disk provided with a target region for emitting an X-ray, a rotary mechanism rotatably supporting the rotary anode disk directly or with a supporting shaft arranged therebetween, and a cathode for irradiating the target region with an electron beam. This rotary anode disk, the rotary mechanism and the cathode are arranged within a vacuum envelope. The rotary mechanism for supporting the rotary anode disk comprises a rotary structure having bearing sections formed between the rotary anode disk and the rotary mechanism and a stationary structure.
In the X-ray tube apparatus comprising the rotary anode type X-ray tube described above, a rotating magnetic field is generated from a stator electromagnetic coil arranged outside the vacuum envelope of the X-ray tube so as to rotate the rotary anode disk jointed to the rotating mechanism at high speed using the principle of an electromagnetic induction motor. As a result, the target region of the rotary anode disk is irradiated with the electron beam generated from the cathode so as to allow an X-ray to be emitted from the target region.
The rotary mechanism of the conventional rotary anode type X-ray tube, which rotatably supports the rotary anode disk, will now be described with reference to FIGS. 1 and 2. As shown in FIGS. 1 and 2, the rotary mechanism comprises a supporting shaft 31. A rotary anode disk (not shown) provided with a target region made of a heavy metal and emitting an X-ray is fixed to the supporting shaft 31. Also, a cylindrical rotor 32 for rotatably supporting the rotary anode disk is coupled with the supporting shaft 31.
The rotor 32 is of a triple coaxial structure consisting of an outer cylinder 32a, an intermediate cylinder 32b, and an inner cylinder 32c having a bottom. The outer cylinder 32a and the intermediate cylinder 32b are brazed to each other to form an integral structure in an upper open region B1 shown in FIG. 1. Incidentally, the upper portion of the intermediate cylinder 32b is bonded directly to the supporting shaft 31.
Further, the intermediate cylinder 32b and the inner cylinder 32c are brazed to each other to form an integral structure in a lower open portion shown in FIG. 1. To be more specific, as apparent from FIG. 2 showing a lateral cross section along the line IIxe2x80x94II shown in FIG. 1, the outer cylinder 32a, the intermediate cylinder 32b and the inner cylinder 32c are arranged coaxial, and the intermediate cylinder 32b and the inner cylinder 32c are integrally bonded to each other by a brazed portion B2 over the entire circumferential region in a lower end portion of the rotary mechanism.
A columnar stator (not shown) is inserted into the inner cylinder 32c of the rotor 32 with a small bearing clearance of about 20 xcexcm provided between the outer circumferential surface of the stator and the inner circumferential surface of the inner cylinder 32c. The intermediate cylinder 32b is formed of, for example, a ferromagnetic material and also performs the function of a magnetism guiding section of the rotary magnetic field generated from a stator electromagnetic coil (not shown).
A heat insulating clearance G1 having a size of, for example, about 0.5 mm in the radial direction is formed between the outer cylinder 32a and the intermediate cylinder 32b. Also, a heat insulating clearance G2 having a size of, for example, about 1 mm in the radial direction is formed between the intermediate cylinder 32b and the inner cylinder 32c. 
During operation of the rotary anode type X-ray tube, the target region of the rotary anode disk is irradiated with an electron beam, with the result that the rotary anode disk is heated to one thousand and several hundred degrees centigrade. The heat of the rotary anode disk is transmitted to the rotor via the supporting shaft, etc. so as to elevate the temperature of the hydrodynamic type slide bearing portion arranged between the inner cylinder 32c and the stator, thereby impairing the rotating characteristics of the rotor.
Such being the situation, the intermediate cylinder 32b that is bonded directly to the supporting shaft is generally formed of a material having a low heat conductivity in order to prevent the heat of the rotary anode disk from being transmitted to the bearing section as much as possible. Also, since heat is generated in the bearing section during operation, it is said to be desirable for the inner cylinder constituting the bearing surface to be formed of a material having a high heat conductivity in order to permit the generated heat to be dispersed and released efficiently to the outside.
As described above, the intermediate cylinder is formed of a material having a low heat conductivity, and the inner cylinder is formed of a material having a high heat conductivity. Naturally, the intermediate cylinder and the inner cylinder are formed of different materials, and the intermediate cylinder and the inner cylinder differ from each other in the thermal expansion coefficient in many cases. It follows that it is difficult in some cases to bond the intermediate cylinder and the inner cylinder by means of brazing.
To be more specific, where these cylinder members are bonded to each other by a welding material, e.g., by a gold brazing, it is necessary to heat the welding material to about 1100xc2x0 C. Also, in the case of silver brazing, the welding material must be heated to about 800xc2x0 C. What should be noted is that, if the intermediate cylinder and the inner cylinder differ from each other in the thermal expansion coefficient, a large difference is generated between the coupled size between the intermediate and inner cylinders at room temperature and the coupled sizes of the intermediate and inner cylinders at brazing temperature.
Suppose, for example, that the thermal expansion coefficient of the intermediate cylinder is higher than that of the inner cylinder. If the brazing is performed under the state that the intermediate and inner cylinders are exactly coupled at room temperature, the inner diameter of the intermediate cylinder is rendered larger than the outer diameter at the brazed portion of the inner cylinder under the high brazing temperature, with the result that it is possible for the intermediate and inner cylinders to be brazed to each other with a non-uniform clearance provided therebetween and with the axes of the intermediate and inner cylinders deviated from each other.
To be more specific, it is certainly possible for the intermediate cylinder and the inner cylinder to be brazed to each other with the axes of these two cylinders substantially aligned. Alternatively, it is also possible for an inconvenience to take place as shown in FIG. 3. To be more specific, it is considered possible for the intermediate and inner cylinders to be brazed to each other with the axis Cr of the intermediate cylinder 32b inclined by a certain angle xcex1 relative to the axis Co of the inner cylinder 32c with respect to the axis of the brazed portion B1.
Where the axes of the inner cylinder and the intermediate cylinder are deviated from each other, it is certainly possible to correct to some extent the unbalanced rotation by the processing after the brazing step. However, where the rotary structure is processed at room temperature, the balance of rotation is rendered poor at the high temperature during operation of the X-ray tube so as to render the rotation characteristics poor. Particularly, in a rotary anode type X-ray tube comprising a hydrodynamic slide bearing for high speed rotation having an angular speed of, for example, 6,000 rpm to 10,000 rpm, it is possible for a slight error in the balance of rotation to bring about a serious problem.
On the other hand, where the intermediate cylinder has a low thermal expansion coefficient, the clearance of the coupled portion, in which the intermediate cylinder and the inner cylinder are brazed to each other, is rendered large at room temperature. As a result, under a cooled state after the brazing step, the inner cylinder is shrunk greatly, with the result that it is possible for the brazed portion of the intermediate cylinder to be locally damaged, e.g., occurrence of cracks. It is also possible for the axes of the intermediate cylinder and the inner cylinder to be deviated from each other.
An object of the present invention is to provide a rotary anode type X-ray tube free from deviation of the axes of two cylindrical rotors coaxially coupled with each other so as to exhibit satisfactory rotating characteristics and an X-ray tube provided with the particular rotary anode type X-ray tube.
According to a first aspect of the present invention, there is provided a rotary anode type X-ray tube comprising a substantially columnar stator; a first cylindrical rotor coupled around the stator; at least one hydrodynamic slide bearing including a spiral groove arranged in the coupling portion between the stator and the first cylindrical rotor; and a second cylindrical rotor arranged coaxial with and outside the first cylindrical rotor with a gap for the heat insulation provided therebetween and bonded to a rotary anode disk including a target region for emitting an X-ray formed in a part thereof, the second cylindrical rotor being bonded to the first cylindrical rotor in an open region positioned remote from the rotary anode disk in terms of the heat transmission route; wherein a plurality of slits extending substantially along the axis of rotation are formed apart from each other in the circumferential direction in that region of the second cylindrical rotor which is bonded to the first cylindrical rotor.
Also, according to a second aspect of the present invention, there is provided a rotary anode type X-ray tube apparatus, wherein a thick portion is formed in the first cylindrical rotor made of a ferromagnetic material or the second cylindrical rotor of the rotary anode type X-ray tube in a manner to partially narrow the gap for the heat insulation formed between the first and second cylindrical rotors, a plurality of slits extending substantially along the axis of rotation are formed apart from each other in the circumferential direction in that region of the second cylindrical rotor which is bonded to the first cylindrical rotor, and the iron core portion of the stator electromagnetic coil is located in the outer circumferential region in the position in the axial direction corresponding to the thick portion.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.